Impossible to narrow it down. I started watching the NBA back in the early 90s, the Jordan era, so I keep going back to those Bulls games against the Knicks, Cavs, or Jazz. Seeing Jordan in full force was just intense.
But other great ones were seeing the 2004 Pistons, an extremely balanced team full of role-players, beating the Super-Lakers. Also, the Dallas Mavs win over the Heat was impressive. There are too many, but I have to give it more thought.